A solid transplantable tumor, a rhabdomyosarcoma, in the rat will be used to study the radiation response produced by heavy ion beams. This tumor system as developed by the Barendsen group in Holland, has the property that cells from the excised tumor can be plated in vitro with acceptable plating efficiences. Thus, inhibition of cellular clonogenic capacity can be assayed, and survival curves for cells irradiated in vivo can be obtained. RBE for various endpoints such as tumor growth delay and cell killing will be measured in the peak regions of selected heavy ion beams. OER will be measured at various points in the beams, using the in vitro cell line. Autoradiographic data will be obtained to study cell kinetic parameters, such as cell cycle time and the fraction of proliferating cells. The data will be studied by means of a recently developed cell kinetic model of tumor growth and regression after irradiation. The model will be expanded to include heavy ion irradiation. Finally, fractionation will be introduced into the model, and fractionation experiments with the heavy ion beams will yield information on the important factors controlling this tumors response to these high LET beams.